spree_rename_photos.rb

    def rename_photos
      images.each do |image|                   
        extension = File.extname(image.attachment_file_name).downcase
        old_file_name = image.attachment_file_name
        new_file_name = slug+"-"+"#{image.position}"+"#{extension}"
        image.attachment_file_name = new_file_name
        (image.attachment.styles.keys+[:original]).each do |style|        
          FileUtils.move(File.join(File.dirname(image.attachment.path(style)), old_file_name), image.attachment.path(style))
        end   
        image.save!
      end
    end